Air treatment, i.e., any of various processes aimed at removing undesired substances from the air, is of great interest especially in human dwellings and workplaces. Attention to the quality of air in rooms and other spaces is increasing from research indicating that breathing purer air provides tangible health benefits. For example, many people live in or otherwise spend large amounts of time in single rooms or other relatively confined spaces in which the air available for breathing can become excessively laden over time with potentially harmful particulates, volatile compounds, and other contaminants. Also, with the increased emphasis on making living spaces and workspaces more environmentally tight, the air contained in such spaces can become more rapidly laden with levels of particulate and volatile contaminants that pose unacceptable health risks to the person or persons who occupy such spaces.
One conventional approach to air treatment is passing the air through a filter. For example, most residential forced-air heating units include a passive filter configured to remove larger, easily entrapped particulates such as pet hair and aggregates of lint and dust, principally for the purpose of protecting the heating equipment (e.g., the blower) from becoming overly burdened with accumulated debris from the air. Air passes through the filter whenever the heating unit is running. A disadvantage of this approach is that these filters have large interstitial spaces to ensure that the filter exhibits a very low pressure drop. As a result, these filters (while being better than no filter at all) are notoriously ineffective in capturing small particulates and volatile contaminants. If the pore size of these filters were reduced sufficiently to capture a large percentage (by count) of particulates in the air passing through the filter, then the filter would have too high a pressure drop (i.e., exhibit too high a flow resistance) to be usable with the forced-air heating unit. Also, filters having very small pore sizes are easily and rapidly clogged due to debris accumulation on upstream surfaces, which causes a rapid decline in the ability of the filters to pass air without having to apply a prohibitively high pressure gradient across the filter.
Another air-treatment approach involves passing the air through a region in which the air is ionized or subjected to generated electrons. This approach utilizes a source of electricity to produce an electrical charge in the region. The charge has sufficient amplitude to generate negative ions from the molecules of air gases exposed to the charge in the region. Particulate contaminants suspended in air, such as dust, smoke, and pollen, are usually made up of small, positively-charged particles. The generated negative ions combine with airborne positively charged particles and electrically neutralize them. The resulting neutral-charged particles fall to the earth or floor under the action of gravity. Thus, “ionized” air tends to reduce the concentration of suspended particles in the air. Unfortunately, most devices that produce ionized air also produce ozone, which has become generally recognized as an undesirable contaminant especially in room air. Ionizers also tend to overcharge airborne particles, thereby rendering them attractive to oppositely charged surfaces. This can result in an increased particulate accumulation on various surfaces in the room such as walls, furniture, and draperies.
Another known type of air treatment, called “photo-ionization,” also produces ozone. In photo-ionization, the air is routed past a light source that produces ultraviolet light at a wavelength (about 185 nm) at which oxygen in the air is ionized to produce ozone. Ozone in sufficient concentration is an effective oxidizer of many types of organic compounds including the compounds that make up biological structures on microorganisms such as bacteria, algae, mildews, and molds. Thus, ozone destructively reacts with these microorganisms, which is effective especially in eliminating odors otherwise caused by them. Unfortunately, photo-ionization is not effective or at most poorly effective in physically removing fine particles such as soot, smoke, animal dander, and certain microorganisms from air. Also, as noted above, producing and discharging ozone into room air is not desirable.
Yet another known type of air treatment involves passing air through a gas-absorbing material such as granules of activated carbon (charcoal), wherein activated carbon is an effective adsorber of gaseous and certain molecular airborne contaminants. Conventional carbon gas-phase filters typically are configured for industrial use, and frequently exhibit any of various undesirable traits such as production of excessive amounts of carbon dust, and short service life. Reducing dust production can be achieved by attaching the granules of activated carbon to a matrix, but many such efforts tend to mask most of the surface of the carbon granules with adhesives or binders, which substantially reduces the effectiveness of the granules.
Hence, effective air treatment poses substantial challenges in the application of effective techniques. Whereas there have been various efforts to combine multiple air-treatment techniques in a single apparatus, these efforts heretofore have yielded disappointing results.